1. Field of the Invention
This invention relates to an acquisition system in an SDMA/TDMA satellite communication system.
2. Description of the Prior Art
Strictly speaking, the SDMA/TDMA satellite communication system is referred to as SDMA/SS-TDMA (Space Division Multiple Access/Spacecraft Switched-Time Division Multiple Access). This system is seen as a satellite communication system of large capacity based on the TDMA system.
With this system, a satellite carries a spot beam antenna which irradiates a relatively small zone, unlike a conventional spot beam antenna. Namely, each spot beam corresponds to one zone without interference with the other beams. The same frequency is employed in common to the respective spot beams and multiple access is achieved. The satellite contains a swithing matrix having respective inputs and outputs corresponding to the spot beam zones. The transmission and reception of signals are achieved among the spot beam zones in accordance with a time sequence predetermined by a clock of a reference oscillator incorporated in the satellite. Further, in each spot beam zone, access is effected on the conventional TDMA system.
FIG. 1 shows a basic model of the SDMA/TDMA satellite communication system (hereinafter referred to as the SDMA/TDMA system, for the sake of brevity).
In FIG. 1, reference numeral 1 indicates a satellite having a switching function; 2 designates a timing signal generator source on the satellite 1; 3 identifies a switching circuit shown in a matrix form; 4 to 9 denote high-directivity antennas; and 10 to 12 represent earth station groups belonging to spot beam zones No. 1 to No. 3, respectively.
The switching circuit 3 is adapted to switch time slots in accordance with a time frame construction as shown in FIG. 2A by a timing signal supplied from the timing signal generator source 2. As shown in FIGS. 2A and 2B, a No. 1 data window of such frame construction, provides a transmitted signal from an earth station of the No. 1 zone (10) to be switched to an earth station of the No. 3 zone (12); a transmitted signal from an earth station of the No. 2 zone (11) is switched to an earth station of the No. 1 zone (10); and a transmitted signal from an earth station of the No. 3 zone (12) is switched to an earth station of the No. 2 zone (11). In a No. 2 data window, signals are switched to be transmitted from the earth station of the No. 1 zone (10) to that of the same zone (10), from the earth station of the No. 2 zone (11) to that of the same zone (11) and from the earth station of the No. 3 zone (12) to that of the same zone (12). Further, in a No. 3 data window, signals are switched to be transmitted from the No. 1 zone (10) to the No. 2 zone (11), from the No. 2 zone (11) to the No. 3 zone (12) and from the No. 3 zone (12) to the No. 1 zone (10). Another sync window is provided and, in its time slot, transmitted signals from the No. 1, No. 2 and No. 3 zones (10, 11 and 12) are switched so as to be respectively returned to them, and are used for synchronization of the earth stations with the satellite.
As described above, the switching circuit 3 in the satellite 1 sequentially changes the respective predetermined time slots. Therefore, for proper communication between the earth stations, it is necessary to correctly synchronize with a desired time slot. Accordingly, in the case where a certain earth station starts transmission, it is especially important to achieve an acquisition control by synchronizing the timing of the earth station with that of the satellite.